Models 651-1, 652-1 & 652-2
Precision Light Choppers

651-1 Light Chopper

652-1 & 652-2 Light Choppers

General
The 650 series of light choppers features a control box with separate chopping heads for remote operation. There are three different models within the range, the 651-1 using a standard dual port chopping head and 652-1 and 652-2 employing a single port micro head for use where space is a prime consideration.

All the models in the range offer precision frequency control via digital push-buttons or by the application of an AC signal to the sync input on the control unit. When used in conjunction with a SIGNAL RECOVERY lock-in amplifier, computer control of the chopping frequency can be achieved by use of the lock-in amplifier’s oscillator as a drive signal for the chopper and controlling the oscillator frequency from the computer. In all cases, frequency accuracy and stability are excellent. A LED indicator on the front panel gives constant indication of frequency lock. This indicator can, however, be extinguished for those measurements that need to be executed in total blackout conditions.

One special feature of the 650 series choppers is the ability to add extension leads (model 653A) to increase the remote distance between the chopping head and the control unit. Up to two additional leads can be employed, each 2 m long, producing a maximum distance of 5.5 m (18 ft).

Both the standard head and the micro heads are fully enclosed designs which serve to reduce errors resulting from external air motion and to minimize the risk of accidental damage to the chopping blades.

Quartz Crystal Frequency Accuracy and Stability
The models  651-1, 652-1 and 652-2, in common with all SIGNAL RECOVERY light choppers, uses a quartz crystal oscillator as their primary frequency standard. The oscillator signals is divided down to yield the required chopper frequency, and then the motor speed is continuous adjusted to phase lock the actual chopper frequency to this required value. The result are choppers with an output frequency as stable as other modern frequency sources.

External Frequency Control
Like many other choppers, the frequency can be controlled externally. However, unlike other units the control is via an applied TTL reference rather than an analog voltage. This means that that the modulation frequency generated is exactly that required, and in the case of the dual-aperture model 651-1 allows it to be used in conjunction with the dual reference modes offered by our model 7265 and 7280 lock-in amplifiers to implement two-channel source compensation experiments. The diagram below shows the model 197 used in this mode but is equally valid when this is replaced by the 651-1. Usually, however, the model 198A is chosen in preference to the models 651-1 or 197 for this application since the two frequencies it generates are relative primes.

Using the Model 197 (651-1 may be substituted) with a Model 7265 to implement a
dual-channel source compensation experiment

Model 651-1
Dual Aperture Light Chopper
The model 651-1 uses the standard head. This is a dual port design allowing two optical beams to be modulated simultaneously at different frequencies, f1 and f2. Two reference (sync) outputs are made available at the control unit, corresponding to f1 and f2, for use with signal recovery processing electronics, such as a lock-in amplifier.

Two small blanking plates are provided with the standard head and one of these allows either the top port or the bottom port to be blanked off to reduce the effect of any stray light passing through an unused path. The second plate provides some measure of beam restriction when using large diameter beams on the higher frequency (and therefore smaller aperture) port. The standard head is supplied complete with a flat base plate to allow the unit to be used in a free standing mode on a bench type surface. A special feature of this support allows the head to be used in the horizontal plane for modulating vertical beams. The base plate can be removed and standard optical mounting supports employed instead if preferred.

Model 651-1 Mechanical Dimensions

Model 651-1 Blade Dimensions

Models 652-1 and 652-2
Dual Aperture Light Chopper
The models 652-1 and 652-2 utilize a micro head. This is a single port modulator for use where space is limited. The former uses a 2-slot chopping disc and the latter an 8-slot disc to achieve a different frequency range. Micro heads can be employed as free standing modulators on a bench type surface, or can be supported on standard mounting hardware.

Models 652-1 and 652-1 Mechanical Dimensions

Model 652-1 Blade Dimensions

Model 652-2 Blade Dimensions

Models 652-1 and 652-2 Specifications

SIGNAL RECOVERY is part of Advanced Measurement Technology, Inc, a division of AMETEK, Inc.
 It includes the businesses formerly trading as EG&G Instruments (Signal Recovery),
EG&G Signal Recovery and PerkinElmer Instruments (Signal Recovery)

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